ZnO (zinc oxide) is a direct-transition-type semiconductor having a band gap energy of 3.4 eV. A Zn-base semiconductor composed of ZnO or having ZnO as a base material is therefore a promising material for light emitting device capable of emitting light in blue to ultra-violet regions. The Zn-base semiconductor light emitting device is, however, fabricated by allowing a light emitting region composed of Zn-base semiconductor to epitaxially grow on the surface of a different kind of substrate such as a sapphire substrate, due to difficulty in obtaining a Zn-base semiconductor single crystal substrate having a desirable quality at low cost, unlike the case of a light emitting device using GaAs-base semiconductor or the like. For this reason, various efforts have been made on improvement in crystallinity of a buffer layer formed between the substrate and the light emitting region, in order to ensure desirable crystallinity of the light emitting region which affects emission characteristics such as emission efficiency, half value width of emission wavelength and so forth.
One example of the Zn-base semiconductor light emitting device intended for improving the crystallinity of the buffer layer is disclosed in Japanese Laid-Open Patent Publication No. 2001-68485, in which a single crystal stack, later converted into a buffer layer, is formed on a sapphire substrate at a temperature lower than the formation temperature of the light emitting region, and the stack is then annealed at a temperature almost equivalent to the formation temperature of the light emitting region, to thereby planarize the surface thereof and make it into the buffer layer.
It is, however, strongly demanded for the formation of the stack, later converted into the buffer layer, in a form of a single crystal layer under the low-temperature condition lower than the formation temperature of the light emitting region as described in the aforementioned Japanese Laid-Open Patent Publication No. 2001-68485, that the conditions such as formation temperature and length of time with respect to the formation must precisely be controlled. It is further necessary to adjust frequency band used for generating radicals when the stack is formed using an RS-MBE (Radical Source Molecular Beam Epitaxy) apparatus as disclosed again in Japanese Laid-Open Patent Publication No. 2001-68485. As is known from the above, formation of the stack to be converted into the buffer layer in a form of single crystal layer requires a high level of accuracy in the process control and causes increase in the cost. First of all, one of essential advantages of the Zn-base semiconductor including ZnO in the industrial application resides in its inexpensiveness as compared with other blue-light-emitting InGaN-base semiconductor or the like. Also from this point of view, it is important subject to fabricate the Zn-base semiconductor at a cost as low as possible.
This invention is conceived taking the above-described subjects into consideration, and an object thereof resides in providing a Zn-base semiconductor light emitting device which can readily be fabricated and capable of improving quality of the light emitting region, and a method of fabricating the same.